The present invention relates to a composite member comprising different members bonded to one another, and more particularly to a composite member comprising different members bonded using a specific solder material through solid phase bonding, and a method for making the composite member.
There is a method of using a solder material for bonding different members, for example, a ceramic base and a metallic member. However, during cooling operation in bonding them at high temperatures, thermal stress is generated owing to the differences in thermal expansion coefficients between the different members or between the member and the solder material used for bonding these different members. Different thermal expansion coefficients between the members can cause separation at the bonded interface, or if one of the members is fragile, cracks occur in the vicinity of the bonded interface and sometimes the desired bonding strength or airtightness cannot be obtained. The products in which these defects are caused during the production steps must be disposed of as rejected products, and this results in an increase of the production cost of these composite members. Moreover, if they are subjected to thermal cycles in use, the defects occur after use of a certain period to cause deterioration of reliability of the products.
When different members are bonded using a solder material, a method is generally employed according to which the surface of the ceramic base to be bonded is plated with a metal such as Ni to ensure wetting between the ceramic base and the solder material, then these members are disposed opposite to each other with a suitable space, the solder material is poured into the space, and the members are bonded. There is another method according to which an additive such as Ti which can ensure wetting by forming a reactive layer of a nitride or an oxide on the surface of the ceramic base is added to the solder material, without carrying out the plating with a metal. However, in these methods, unless some means are adopted for reducing the thermal stress generated at the bonded part, cracks are often formed on the side of the ceramic base which is fragile against thermal stress or separation occurs at the bonded part to adversely affect various performances such as bonding strength and airtightness required for composite members. It is especially difficult to bond a member of low strength, such as aluminum nitride to a different member such as a metallic material while inhibiting the occurrence of the above problems.
In order to solve the above problems, there can be considered a method of bonding a base and a metallic member by liquid phase bonding with a solder material comprising only a metal of low proof stress such as Au which undergoes plastic deformation by a low stress. However, in the case of this method, if Ni, Co, Kovar or the like is used as the metallic member, the component (Fe, Ni, Co) diffuses into Au to cause increase of proof stress of the solder material, and as a result, the residual stress at the bonding cannot be completely absorbed by the plastic deformation of the solder material, and cracks occur in the ceramic base at the finishing of bonding or due to thermal cycle and thermal shock applied after the finishing of bonding.
Furthermore, it is known that when Au-18Ni solder material and an electrical conductor (Mo) are bonded, Ni in the solder material reacts with Mo to form a fragile texture. Therefore, when use of them as members for high-temperature heaters is supposed, endurance characteristics are deteriorated when the bonded part is exposed to thermal cycle and thermal shock, and, furthermore, the bonded part is rapidly deteriorated.
Moreover, when, for example, Kovar is used as the metallic member, the components constituting the Kovar (Fe, Ni, Co) diffuse into the solder material to form an intermetallic compound layer of low electrical conductivity, which causes deterioration of thermal cycle characteristics and abnormal heat generation at that part.
On the other hand, use of a metal which does not form solid solution with Au as the metallic member can also be considered, and W, Mo and the like can be mentioned as metallic materials which meet the requirement. However, these metallic materials are severely oxidized under high temperature conditions in the air, and cannot be used as metallic members for high-temperature heaters which are exposed to such conditions.
For solving the above problems, it has been attempted to devise the bonding structure. For example, JP-A-10-209255 discloses a bonding structure of a ceramic base and a connector for power supply which have the structure shown in FIG. 4 as a susceptor for disposing a semiconductor wafer. In FIG. 4, a hole 14 is provided in a ceramic base 1. In the hole 14, a metallic member 17 such as Mo is exposed which is previously embedded in the ceramic base 1 and has a thermal expansion coefficient approximate to that of the ceramic base 1. Furthermore, a cylindrical atmospheric protector 9 is inserted in the hole 14. Inside the atmospheric protector 9, there are inserted a power supplying connector 16 and a low thermal expansion material 15 for stress relaxation. The atmospheric protector 9 and the connector 16 are airtightly bonded with a solder material 5, and the low thermal expansion material 15 and the atmospheric protector 9 are airtightly bonded with solder material 5 in respect to the metallic member 17.
According to this bonding structure, the low thermal expansion material 15 and the metallic member 17 are relaxed in residual stress at bonding and, besides, oxidation of the metallic member 17 such as Mo is inhibited by the atmospheric protector 9, and, hence, even if bonding is performed using a solder material of high proof stress, such as above-mentioned Au-18Ni solder, cracks are not formed in the ceramic base 1 at bonding and furthermore endurance reliability is high in case the bonded part is exposed to thermal cycle and thermal shock at the time of using a high-temperature heater. However, the above bonding structure suffers from the problems that the number of parts increases, and very high production control capacity is required because the metallic member 17 is deteriorated due to oxidation unless the atmospheric protector 9 and the metallic member 17 are completely bonded.
Furthermore, JP-A-11-278951 discloses a bonded member and a bonding method as illustrated in FIGS. 5-7. Enclosed within chamber 21 of housing 20 is a ceramic susceptor 22, which has a wafer placing face 22a and a backside 22b. A wafer 24 is placed on wafer placing face 22a and a corrosion resistant metal ring 23, such as of Kovar, is attached at an interface portion 25 between susceptor 22 and metal ring 23 to back side 22b of ceramic susceptor 22 in the ceramic base having a structure shown in FIG. 5. These member structures are allowed to have the shapes as shown in FIG. 6 and FIG. 7 for relaxation of the generated thermal stress. That is, to allow the member structures to have these shapes is effective for thermal stress relaxation, but in the case of the ceramic being fragile, the solder material changes in properties due to dissolution of the metallic member in the method of bonding the metallic member and the ceramic base by melting the solder material as disclosed in the above patent publication, and the effect to relax the thermal stress is insufficient with giving only such care for the bonding structure as disclosed in the above patent publication, resulting in troubles such as rupture of the ceramic base.
The present invention has been made in view of the problems in the conventional techniques, and the object is to provide a composite member and a method for making the composite member, according to which a ceramic base having both the thermal cycle characteristics and the thermal shock characteristics and a metallic member are bonded to each other.
According to the present invention, there is provided a composite member made by bonding different members, namely, a ceramic base and a metallic member, characterized in that an active metal foil is disposed on the surface of the ceramic base and a solder material comprising Au is disposed on the active metal foil, the active metal foil and the solder material are heated to form a bonding layer, the metallic member is disposed on the surface of the bonding layer, and these are pressed and heated to bond the bonding layer and the metallic member through solid phase bonding.
Furthermore, according to the present invention, there is provided a composite member made by bonding different members, namely, a ceramic base and a metallic member, characterized in that an active metal foil is disposed on the surface of the ceramic base and a solder material comprising an Auxe2x80x94Ag alloy is disposed on the active metal foil, the active metal foil and the solder material are heated to form a bonding layer, the metallic member is disposed on the surface of the bonding layer, and these are pressed and heated to bond the bonding layer and the metallic member through solid phase bonding. In the present invention, it is preferred that the proportion of Ag contained in the Auxe2x80x94Ag alloy is 0.5-80 wt %.
Moreover, in the present invention, it is preferred that an electrical conductor comprising Mo, W or an alloy of Mo and W is embedded in the ceramic base with a part of the surface of the electrical conductor being exposed to the exterior of the ceramic base, and that the material of the ceramic base is at least one member selected from the group consisting of aluminumnitride, siliconnitride, alumina, zirconia, magnesia, spinel, and silicon carbide.
In the present invention, it is preferred that the material of the metallic member is at least one metal selected from the group consisting of Ni, Co, Fe and Cr, and it is also preferred that the material of the metallic member is an alloy having as main constituting elements at least one metal selected from the group consisting of Ni, Co, Fe and Cr.
Furthermore, in the present invention, it is preferred that the material of the active metal foil is at least one metal selected from the group consisting of Ti, Nb, Hf and Zr. The composite member made by bonding different members according to the present invention is suitably employed as a susceptor for disposing a semiconductor wafer.
On the other hand, according to the present invention, there is provided a method for making a composite member by bonding different members, namely, a ceramic base and a metallic member, characterized by including a first step of disposing an active metal foil on the surface of the ceramic base and a solder material comprising Au on the active metal foil and heating them to form a bonding layer comprising the solder material on the surface of the ceramic base, and a second step of disposing the metallic member on the surface of the bonding layer and pressing and heating them to bond the bonding layer and the metallic member through solid phase bonding.
In addition, according to the present invention, there is provided a method for making a composite member by bonding different members, namely, a ceramic base and a metallic member, characterized by including a first step of disposing an active metal foil on the surface of the ceramic base and a solder material comprising an Auxe2x80x94Ag alloy on the active metal foil and heating them to form a bonding layer comprising the solder material on the surface of the ceramic base, and a second step of disposing the metallic member on the surface of the bonding layer and pressing and heating them to bond the bonding layer and the metallic member through solid phase bonding. In the present invention, it is further preferred to use an Auxe2x80x94Ag alloy in which the proportion of Ag is 0.5-80 wt %.
In the present invention, it is preferred to use a ceramic base in which an electrical conductor comprising Mo, W or an alloy of Mo and W is embedded in the ceramic base with a part of the surface of the electrical conductor being exposed to the exterior of the ceramic base, and it is preferred to use a ceramic base comprising at least one material selected from the group consisting of aluminum nitride, silicon nitride, alumina, zirconia, magnesia, spinel and silicon carbide.
Furthermore, in the present invention, it is preferred to use a metallic member comprising at least one material selected from the group consisting of Ni, Co, Fe and Cr, and it is also preferred to use a metallic member comprising an alloy having, as main constituting elements, at least one material selected from the group consisting of Ni, Co, Fe and Cr.
In the present invention, it is preferred to use an active metal foil comprising at least one material selected from the group consisting of Ti, Nb, Hf and Zr.